feat(engine,server): XP toast on win + display_name max-length validation

ProgressPlugin now fires XpAwardedEvent on every win. AnimationPlugin
shows a "+N XP" toast so players see XP feedback immediately after
winning. Server leaderboard opt-in endpoint also now validates that
display_name is at most 32 characters.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_engine/src/animation_plugin.rs

@@ -11,7 +11,7 @@ use crate::auto_complete_plugin::AutoCompleteState;
 use crate::card_plugin::CardEntity;
 use crate::challenge_plugin::ChallengeAdvancedEvent;
 use crate::daily_challenge_plugin::{DailyChallengeCompletedEvent, DailyGoalAnnouncementEvent};
-use crate::events::{InfoToastEvent, NewGameConfirmEvent};
+use crate::events::{InfoToastEvent, NewGameConfirmEvent, XpAwardedEvent};
 use crate::events::{AchievementUnlockedEvent, GameWonEvent};
 use crate::game_plugin::GameMutation;
 use crate::layout::LayoutResource;
@@ -94,6 +94,7 @@ impl Plugin for AnimationPlugin {
             .add_event::<SettingsChangedEvent>()
             .add_event::<NewGameConfirmEvent>()
             .add_event::<InfoToastEvent>()
+            .add_event::<XpAwardedEvent>()
             .init_resource::<EffectiveSlideDuration>()
             .add_systems(Startup, init_slide_duration)
             .add_systems(
@@ -113,6 +114,7 @@ impl Plugin for AnimationPlugin {
                     handle_auto_complete_toast,
                     handle_new_game_confirm_toast,
                     handle_info_toast,
+                    handle_xp_awarded_toast,
                     tick_toasts,
                 )
                     .after(GameMutation),
@@ -330,6 +332,12 @@ fn handle_info_toast(mut commands: Commands, mut events: EventReader<InfoToastEv
     }
 }
 
+fn handle_xp_awarded_toast(mut commands: Commands, mut events: EventReader<XpAwardedEvent>) {
+    for ev in events.read() {
+        spawn_toast(&mut commands, format!("+{} XP", ev.amount), 3.0);
+    }
+}
+
 fn tick_toasts(
     mut commands: Commands,
     time: Res<Time>,